CEPT Europa
CEPT is the European Conference of Postal and Telecommunications Administrations. You will see that acronym on many European stamps. Its work was superseded by Europa. Each year since the founding in 1956, nations have issued stamps of a common theme. (Here’s a list of those themes. This Wikipedia article also lists the themes.) I have published a more comprehensive story on this group.
Postage Stamp Nation
This has nothing to do with size. Rather, it’s a country that issues postage stamps as a major source of revenue. Many African nations do this: Burundi, for example, has issued 8,234 stamps since it became a nation in 1962, including 1,685 in 2012 alone. These tend to be on topics that are of interest to topical collectors. I try not to use stamps from these nations for my stories, but sometimes the story is compelling and the PSN is the only source. (This information is from colnect.com, a wonderful source of information about worldwide stamp.)